How to decide whether Kd = [NH3(aq)] / [NH3(CHCl3)] or Kd = [NH3(CHCl3)] / [NH3(aq)] when either upper layer/lower layer or density is not given?In short when i have no idea on the density how can i decide what is the upper layer and what is the lower layer?

No general convention exists regarding whether the concentration in the organic phase or aqueous phase should be placed in the numerator to determine K_d. One should always specify which way the partition coefficient is being expressed.

e.g. K_d=C_(water)/C_(CHCl3)=2

However, while purifying a particular compound from a mixture through liquid-liquid extraction, the impure mixture is dissolved in a small volume of solvent A first, then the desired compound is extracted with solvent B, in which the desired compound is more soluble. In that case, the convention is to express the partition co-efficient as (C_B)/(C_A). In other words, concentration of the compound in the extracting solvent, or the solvent that is used repeatedly in large quantities, is placed at the numerator.

Choice of procedure based on a knowledge of relative solubility, rather than density is the key here.